Often a manufacturer will warranty a product used outdoors for a specified lifetime. Accelerated weathering devices are used by a number of industries to test a product's performance after exposure to outdoor environmental conditions such as temperature cycles, moisture including humidity, condensation, and rain, as well as exposure to (i.e. simulated) solar radiation. Although all such stresses can result in product degradation, exposure to solar radiation is one of the more influential factors in weathering because ultraviolet rays often tend to break down polymers and other materials over time.
Among the more difficult tasks in the manufacture of accelerated weathering devices is to provide a spectral power distribution of artificial light that matches closely to that of natural sunlight at the earth's surface, i.e. terrestrial sunlight. This is typically accomplished by passing illumination from an artificial light source through one or more optical filters to filter out wavelengths of light that are not present in actual sunlight. By matching or closely approximating the spectral power distribution of natural sunlight, the results of exposure to the accelerated weathering device will more closely approximate effects of real world exposure.
Commercially available optical filters often pass more amounts of ultraviolet radiation, at wavelengths less than 290 nm, than present in actual terrestrial solar radiation. Radiation at these wavelengths can cause materials, such as polyethylene terephthalate and polyurethanes prepared from phthalate containing polyols, to fail prematurely. Other commercially available optical filters transmit insufficient intensity of light within the solar spectrum. This can lead to slower degradation rates or longer test times.
U.S. Pat. No. 4,931,655 describes and apparatus for accelerated weather testing of a sample using a metal halide lamp. The optimum filter is described as being made of soft glass having a low melting point which consists of, for instance, SiO2 of 60-65% (percentage by weight), Pb of 15-20%, Na of 7-8%, K of 7-8%, Co of 1%, and Ni of 1%.
U.S. Pat. No. 6,859,309 is directed to an optical filter for use in accelerating weathering device that exhibits certain irradiance ratios that approximate sunlight. A preferred optical filter includes a glass having a lead content of between 0.5% and 50% by weight that is free of visible light absorbing components. In some examples, the filter can be constructed to have a thickness of 0.7 mm to 10 mm. In another aspect, the optical filter is part of an optical filter that may further include an ultraviolet transmissive optical filter. The ultraviolet transmissive optical filter may be constructed from quartz glass and may further include an infrared absorbing coating.